In recent years, a system for performing power line communication has been in practical use. In the power line communication, high-frequency signals are superimposed on a power line (also referred to as a distribution line). The power line communication is performed using an existing power line without the need for installation of dedicated wiring to build an indoor communication network, and therefore is expected to become increasingly popular. Power distribution systems, however, are normally not designed for use in communication. Therefore, the power line communication can cause an impedance mismatch to a line (wiring) at an end of the power distribution system (socket). Once the impedance mismatch is caused, reflection of high-frequency signals used in the power line communication occurs at a point where the mismatch is caused. The reflection of high-frequency signals leads to frequency selective fading that can result in a problem of channel characteristic degradation in the power line communication.
As a power line for use in interior distribution, a power line having characteristic impedance on the order of approximately 100 ohms is generally used.
When a household electrical appliance having impedance of a few ohms to high-frequency signals is connected to a socket at the end of the power distribution system, the impedance of the socket becomes extremely lower than the characteristic impedance of a power line. On the other hand, when no electronic equipment is connected to the socket, the end becomes an open end. In such a case, the impedance of the socket becomes extremely higher than the characteristic impedance of the power line. Accordingly, a mismatch occurs between the impedance to high-frequency signals in a case where the socket is the open end and the impedance to the high-frequency signals in a case where the socket is not the open end, at the end of the power distribution system (socket). As a result, intense reflection of high-frequency signals occurs, and thus channel characteristic in the power line communication is degraded.
In order to prevent such channel characteristic degradation, a termination circuit for reducing the impedance mismatch at the socket or the like is disclosed (see Patent Literature 1).
FIG. 25 illustrates a configuration of the termination circuit disclosed in Patent Literature 1. The configuration of the termination circuit illustrated in FIG. 25 is as follows: an inductor 2503 is directly connected to a line (power line), and a circuit including a termination resistor 2501 and a capacitor (condenser) 2502 connected in series is connected in parallel with the line. Here, as the termination resistor 2501, a resistor having resistance approximately equal to the characteristic impedance of the line is used. Used as the capacitor 2502 is a capacitor having sufficiently lower impedance than the characteristic impedance of the line to a high-frequency signal for use in the power line communication, and having sufficiently higher impedance than the characteristic impedance of the line to a power current (e.g. an alternating current of 50 Hz and 60 Hz, and a direct current). Used as the inductor 2503 is an inductor having sufficiently higher impedance than the characteristic impedance of the line to the high-frequency signal, and having sufficiently lower impedance than the characteristic impedance of the line to the power current.
The high-frequency signal from a line (wiring) side is provided to the circuit including the termination resistor 2501 and the capacitor 2502 connected in series, and output from a signal input-output terminal 2504. The line (wiring) side and a socket side, to which electronic equipment is connected, are separated from each other by the inductor 2503 having high characteristic impedance to the high-frequency signal. Therefore, even when a high-frequency impedance of the electronic equipment connected to the socket is low, the effect thereof is negligible. As described above, the termination circuit disclosed in Patent Literature 1 has impedance approximately equal to the characteristic impedance of the line to the high-frequency signal, regardless of whether or not any equipment is connected to the socket. Therefore, the impedance mismatch at the end is reduced, and reflection of the high-frequency signals is suppressed.